Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction peripheral having a plurality of these functions.
Description of the Related Art
With the high speed of an image forming apparatus, as a full color image forming apparatus, the configuration in which a plurality of image forming units are disposed side by side along the belt member and respective color image forming operations are processed in parallel has become the mainstream. For example, an electro-photographic image forming apparatus employs a full color tandem system using an intermediate transfer belt.
In the full color tandem system, a plurality of image forming units disposed side by side are used. The toner images having different colors formed by these image forming units are primarily transferred to the intermediate transfer belt in an overlapping manner. Furthermore, after the toner images which have been transferred to the intermediate transfer belt are secondarily transferred to the recording material altogether, a full color image is obtained by fixing the toner images on the recording material.
In the image forming unit, the surface of the photosensitive drum is charged by a charging roller. The surface of the charged photosensitive drum is exposed by a laser scanner and a latent image is formed on the surface of the photosensitive drum. Then, the latent image on the surface of the photosensitive drum is developed into a toner image by a developing device. The developed toner image is transferred to an intermediate transfer belt by a primary transfer roller. Further, the toner remaining on the photosensitive drum after the primary transfer is removed by the cleaning member.
For the image forming apparatus of a full color tandem system, it is necessary to suppress the short life of the image forming unit of a color that is not used during the monochrome image formation. Therefore, in a general configuration of the image forming apparatus, a full color mode in which the intermediate transfer belt is in contact with all the image forming units and a monochrome mode in which the intermediate transfer belt is in contact only with the image forming unit of black are switched (see, for example, U.S. Patent Application Publication No. 2002/018673 A1).
Meanwhile, in such an image forming apparatus, when performing a print job in which a full color image and a monochrome image are mixed, there is a problem that a downtime associated with the switching of the full color mode and the monochrome mode frequently occurs.
Japanese Patent Laid-Open No. 2004-246571 discloses the configuration in which when the printing time can be shortened if a monochrome image is printed using a full color mode, the monochrome image is processed as a full color image.
Japanese Patent Laid-Open No. 2007-11151 discloses the configuration in which developing devices for color are stopped when a monochrome image is printed using a full color mode so that the short life of the image forming units for color is suppressed without generating a downtime.
However, as in the prior art described above, when the images are formed by a part of the developing units with the other developing units being stopped, the following problem occurs.
That is, a study by the inventor of the present invention reveals that when a developing unit is stopped and only the photosensitive drum is rotated, the toner which is present in the opposed portion between the developing unit which is stopped and the photosensitive drum is scraped off by the photosensitive drum and the so-called “fogging image” occurs. This phenomenon will be described later in detail.